You should have known that theElectron Affinity or Electron Gain Enthalpy of Chlorine is higher than that of Flourine.
It is because both the 2p and 3p orbitals, respectively, of Flourine or Chlorine, have 7 electrons. The 2p orbital, which is much smaller and more compact, has a high electron density around Flourine. To enter the atom, an incoming electron must overcome the repulsion. The EA of flourine decreases slightly. Chorine, on the other hand, has a lower EA value than flourine.
The same principle applies for Oxygen, Sulpur, Nitrogen, and Phosphorous pairs.